“…A detailed discussion of Schrödinger operators with δ and δ ′ -point interactions can be found in the standard monograph [13]. For further reading on Schrödinger operators with singular potentials we refer the reader to, e.g., [16,17,24,25,26,29,30,31,32,33,38,40,41,42,43,44] and the references therein.…”